Microdialysis coupled with an enzyme-based flow injection analysis was used to monitor brain extracellular lactate and glucose in the freely moving rat. Glucose levels reflect the balance between supply from the blood and local utilisation, and lactate efflux indicates the degree of local nonoxidative glucose metabolism. Local application of tolbutamide, a blocker of the ATP-sensitive potassium channel, decreased extracellular glucose and lactate levels in the hippocampus but not in the striatum. The increase in glucose and lactate levels following mild behavioural stimulation was also reduced by tolbutamide in the hippocampus. Similar effects on both basal and stimulated lactate levels were obtained with local application of 10 mM glucose. These results indicate that ATP-sensitive potassium channels are active under physiological conditions in the hippocampus and that the effects of tolbutamide can be mimicked by physiological glucose levels.